The present invention relates to improvements in a zoom lens-barrel which may be combined with various types of cameras.
First, referring to FIG. 12, an example of the structure of a conventional zoom lens will be explained.
A zoom lens-barrel 10 of the conventional example is composed as follows. Fixed lens groups comprise the first lens group L.sub.1 which is located in the leading position on the photographic object side, and the third lens group L.sub.3 which is located in the third position. Movable lens groups comprise the second lens group L.sub.2 (referred to as a variator, hereinafter) which is located in the second position, and the fourth lens group L.sub.4 (hereinafter referred to as a master lens) which is located in the last position. Zoom lens system Z' provided inside the lens-barrel is composed of the aforementioned lens groups, an automatic iris mechanism, and a drive system.
During variation of magnification, the in-focus state of zoom lens system Z' can be obtained in such a manner that: variator L.sub.2 moves on an optical axis along curve A shown in FIG. 13; and master lens L.sub.4 moves simultaneously on the optical axis in accordance with a moving diagram which is set between curves B.sub.1 -B.sub.2 shown in FIG. 13 corresponding to an object distance.
In this case, variator L.sub.2 is driven by a zoom motor 51 which will be described later, and the position of variator L.sub.2 on the optical axis is detected digitally by an encoder 61. Master lens L.sub.4 is driven by a stepping motor 71, and the position of master lens L.sub.4 on the optical axis is detected in the form of the number of pulses by a reference position detection device 81.
In order to control the movement of variator L.sub.2 and master lens L.sub.4, an actual zoom lens drive system is composed in such a manner that: the aforementioned curve A is previously converted into a numerical division of the encoder 61, and the aforementioned curves B.sub.1 -B.sub.2 are previously converted into a pulse number which is counted from a reference position; the aforementioned dividing number and pulse number are arranged in the form of a map (or a table), and stored in a variable magnification control system so that they can be used for controlling the movement of variator L.sub.2 and master lens L.sub.4.
On the other hand, a focusing operation is conducted in such a manner that: master lens L.sub.4 is wobbled in the direction of the optical axis by an appropriate means; at the same time, a high frequency component is outputted from an image sensor (not shown in the drawing) located on the focal surface side; as illustrated in FIG. 14 which explains the principle, the level variation of this high frequency component is detected so that the front or rear focus can be judged; and master lens L.sub.4 is moved to a position in which the peak high frequency component can be obtained.
The zoom lens-barrel 10 which holds the aforementioned 4 lens groups L.sub.1 -L.sub.4 is composed of a fixed barrel 2 which includes the front half of the lens-barrel, and a mount member 3 which includes the rear half of the lens-barrel.
In this case, a presser ring 4a is installed in the front end of the fixed barrel 2, and the first lens group L.sub.1 is directly fixed to the fixed lens-barrel 2 by the presser ring 4a.
On the inner circumferential surface of the fixed lens-barrel 2 between an intermediate wall 2a of the fixed lens-barrel 2 and an E-ring 4b, a cam barrel 11 is provided in such a manner that: it can be rotated around the optical axis; and it can not be moved in the direction of the optical axis.
A cam groove 12 is formed on the inner circumferential surface of the cam barrel 11 so that variator L.sub.2 can be moved along a predetermined curve A. In the end of the cam barrel 11, a gear section 13 is formed which is connected with a drive gear 52 of a zoom motor 51.
Inside the cam barrel 11 between an inner circumferential intermediate wall 2a and the E-ring 4b, a zoom guide rail 14 and a rotating motion restricting rail 15 are disposed in parallel to optical axis O.
Variator L.sub.2, which is one of the movable lens groups, is supported by a variator holding frame 21.
This variator holding frame 21 is composed in such a manner that: a bush 22 which is installed above the variator holding frame 21, is slidably engaged with a zoom guide rail 14; a U-shaped groove 23 which is formed in the lower position of the variator holding frame 21, is engaged with the rotating motion restricting rail 15; and a cam follower 24 which is formed in the upper portion of the bush 22 is precisely engaged with the aforementioned cam groove 12.
Therefore, when the cam barrel 11 is rotated during the variation of magnification, the variator holding frame 21 (variator L.sub.2) is driven by the cam follower 24 which moves along the cam groove 12, and moved in the optical axis direction without being rotated.
Positional information of the variator holding frame 21 on the optical axis, can be detected by an encoder 61 which is connected with a gear section 13 of the cam barrel 11, wherein the positional signal is digitally converted into a rotational number (a rotational angle). In this case, the encoder 61 and the gear section 13 of the cam barrel 11 are connected with each other through appropriate gear trains 62, 63.
On the other hand, the mount member 3 which composes the rear half of the lens-barrel, is fixed to the fixed barrel 2, wherein an iris diaphragm group 92 of an appropriate automatic iris diaphragm 91 is disposed between the mount member 3 and the fixed barrel 2, and the third lens group L.sub.3 is directly fixed to the mount member 3. A master lens holding frame 41 can be guided straight along optical axis O when a bush 42 provided below the master lens holding frame 41 is engaged with a master lens guide rail 31 which is installed in parallel with the optical axis between a front wall 3a and a rear wall 3b of the mount member 3.
An engaging rail 43 provided in the upper portion of the master lens holding frame 41 is precisely engaged with a V-shaped groove 34 formed on an upper wall 3c of the mount member 3, so that the rotating motion of the master lens holding member 41 can be prevented.
As described above, the master lens holding frame 41 can be moved in the direction of the optical axis by an appropriate stepping motor 71.
That is, a lead screw 72 formed on the output shaft of the stepping motor 71 is screwed into a nut 73, and a protrusion 73a formed on the outer circumference of the nut 73 is engaged with a groove 33 of a rubber engaging member provided in the master lens holding frame 41, so that the rotation of the nut 73 can be prevented. An arm 44 provided in the master lens holding frame 41 is pushed by a coil spring 35 so that it can be always pressed against the edge surface of the nut 73.
Therefore, when the lead screw 72 is rotated by the stepping motor 71, the nut 73 is moved in the direction of the optical axis, and the arm 44 (the master lens holding frame 41) follows the motion of the nut 73 and moves in the direction of the optical axis.
In this case, the coil spring 35 has the function of eliminating backlash between the lead screw 72 and the nut 73, and the function of eliminating play in the axial direction between the stepping motor 71 and the lead screw 72.
The master lens holding frame 41 is moved under an open loop condition by the numerically controlled stepping motor 71, so that it is necessary to set a reference position. In order to detect the reference position, a reference position detecting device 81 (for example, a photo-interrupter) having an appropriate structure is provided above the upper wall 3c of the mount member 3.
In the aforementioned conventional zoom lens-barrel, there are numerous disadvantages which will be described.
Since the cam barrel 11 is utilized in the conventional lens-barrel, the outer diameter of the fixed barrel 2 is approximately twice as large as the effective diameter of the lens, so that it is difficult to reduce the size of the zoom lens-barrel. The shape of the cam barrel 11 is cylindrical, so that the fixed barrel 2 in which the cam barrel 11 is provided, must be cylindrical, too. In order to house the cam barrel, two parts, the fixed barrel 2 and the mount member 3, are necessary, so that it is difficult to reduce the number of parts.
However, the shape of components (for example, a motor) which compose zoom lens system Z', are not necessarily cylindrical. Consequently, when these components are installed inside the fixed barrel 2, utilization of space inside the fixed barrel 2 is extremely low, which is one of the reasons why the zoom lens can not be made compact.
As illustrated in FIG. 15, in the case of the conventional lens-barrel, various parts such as lens groups L.sub.1 -L.sub.4 composing the zoom lens system must be sequentially assembled in the direction of optical axis O. This assembling work is very complicated. Accordingly, the cost is increased, and further when the assembly of the zoom lens-barrel is automated, it causes serious problems.
Furthermore, in the case of the conventional lens-barrel, high accuracy must be maintained when parts such as a rotary cam, which compose the drive system, are machined. Therefore, it is difficult to reduce the cost.
These problems are common between a zoom lens-barrel and a varifocal lens-barrel in which a plurality of focal distances can be selected, so improvement has been desired for a long time.